It is often desirable to create power quality disturbances on alternating current systems. Such disturbances can be used, for example, to test the immunity of newly designed systems. For example, the SEMI-F47 standard, published by the industry association Semiconductor Equipment and Materials International, and the associated SEMI-F42-0600 testing standard, require that all semiconductor manufacturing equipment tolerate voltage sags to 50% of nominal for 200 milliseconds, to 70% of nominal for 500 milliseconds, and to 80% of nominal for 1 second. Devices that generate such sags for testing purposes are known as sag generators.
Transformer-based sag generators are well-known in the art. Grady et al. in U.S. Pat. No. 5,886,429 and Rockfield et al. in U.S. Pat. No. 5,920,132 disclose typical transformer-based sag generators. However, sag generators must be brought to the test location, so a key requirement for sag generators is portability. Transformer-based sag generators are heavy and awkward to transport.
Amplifier-based sag generators are also well-known in the art. SEMI F42-0600 in its "Related Information 1--Sag Generators" section discusses sag generators that consist of a power amplifier connected to a signal generator. However, such sag generators by their nature require multiple power conversions from alternating current to direct current and back to alternating current, with each conversion having power losses. Most implementations require transformer isolation. For these reasons, amplifier-based sag generators are generally limited to low power applications, and are often even heavier and more awkward to transport than transformer-based sag generators of equivalent output power.
In a sag generator, it is desirable to provide computer-controlled depth and duration of a sag. It is also, in many cases, desirable to provide a computer-controlled envelope of the sag, i.e. to allow the sag depth to vary in a controlled way during the sag. And it is also desirable to provide computer control of the phase angle at which the sag commences.
Typically, sag generators are rated by their nominal voltage and their maximum continuous current, for example 480 volts and 100 amps. Typically, portability of a sag generator is limited when it weighs more than 100 pounds: it becomes difficult to check as luggage on an airplane, and it becomes difficult for an individual to transport and set up.